Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver image, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith a soluble silver complex is obtained by reaction of a silver halide solvent with unexposed and undeveloped silver halide of said emulsion. The photosensitive silver halide emulsion is developed with a processing composition which may be spread between the photosensitive element comprising the silver halide emulsion and a second element which may comprise a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneous therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is at least in part transported in the direction of the print receiving layer and the silver thereof is precipitated in the silver precipitating element to form a positive image.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color such as red or green or blue and by viewing the reversed or positive silver image formed by transfer to a transparent print receiving element through the same or a similar screen which is suitably registered with the positive image carried in the print receiving layer. As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,426; 3,615,427; 3,615,428; 3,615,429; and 3,894,871.
U.S. Pat. No. 4,056,392, issued Nov. 1, 1977, is directed to a diffusion transfer film unit which comprises, in order, an additive color screen, a layer comprising silver precipitating nuclei, a layer comprising a water-soluble cupric salt and a compound selected from the group consisting of chitosan and 2-amino-2-deoxyglucose, and a photosensitive silver halide emulsion layer. By employing a water-soluble copper salt in the chitosan protective layer, an increase in D.sub.max is achieved with substantially no adverse effect on D.sub.min compared to a protective layer composed of chitosan alone.
Enhanced image stability can be provided to silver images by the employment of noble metal compounds containing a noble metal below silver in the Electromotive Series of Elements. A preferred system also includes the employment of an .alpha.,.beta.-enediol silver halide developing agent. Film units and processes disclosing and claiming such stabilization are set forth in U.S. Pat. Nos. 3,704,126, issued Nov. 38, 1972; 3,730,716, issued May 1, 1973; and 3,821,000, issued June 28, 1974.
U.S. Pat. No. 4,279,983, issued on June 21, 1981, is directed to photographic film units and processes employing a noble metal compound adapted to provide enhanced stability to a silver image, said noble metal compound or complex being substantially non-diffusible until the ligand is hydrolyzed, said compound consisting of a noble metal complexed with a ligand adapted to hydrolyze in aqueous alkali to provide a diffusible complex of said noble metal. The noble metal compound may be disposed in various locations in the film unit including a layer adjacent to the layer in which the image silver is to be located.
The preferred compound is represented by the formula EQU X--S--(CH.sub.2).sub.n --COOR
wherein X is a noble metal more noble than silver, n is 1, 2 or 3 and R is an alkyl or substituted alkyl group which may be removed by alkaline hydrolysis.